Reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for upgrading virgin or cracked naphthas for the production of high octane gasoline. Reforming is defined as the total effect of the molecular changes, or hydrocarbon reactions produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins to produce gas and coke. Historically, noble metal catalysts, notably platinum supported on alumina, have been employed for this reaction. More recently, polymetallic catalysts consisting of platinum-rhenium, platinum-iridium, platinum-tin, or various combinations thereof promoted with any one or more of the following elements copper, selenium, sulfur, chloride, and fluoride, have been utilized.
In a typical process, a series of reactors are provided with fixed beds of catalyst which receive downflow feed, and each reactor is provided with a preheater or interstage heater, because the desirable reactions which take place are endothermic. A naphtha feed, with hydrogen, or recycle gas, is cocurrently passed through a reheat furnace and reactor, and then in sequence through subsequent heaters and reactors of the series. The vapor effluent from the last reactor of the series is a gas rich in hydrogen, which usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production; coke invariably forming and depositing on the catalyst during the reaction.
Essentially all petroleum naphtha feeds contain sulfur, a well known catalyst poison which can gradually accumulate upon and poison the catalyst. Most of the sulfur, because of this adverse effect, is generally removed from feed naphthas, e.g., by hydrofining with conventional hydrodesulfurization catalysts consisting of the sulfides of cobalt or nickel and molybdenum supported on a high surface area alumina. The severity of hydrofining can be increased so that essentially all the sulfur is removed from the naphtha in the form of H.sub.2 S. However, small quantities of olefins are also produced. As a consequence, when the exit stream from the hydrofiner is cooled, sulfur can be reincorporated into the naphtha by the combination of H.sub.2 S with the olefins to produce mercaptans. Hence, if a refiner is willing to pay the price, a hydrofiner can be employed at high severity to remove nearly all of the sulfur from a feed, but it is rather costly to maintain a product which consistently contains less than about 1-2 parts per million by weight of sulfur, and of course, during hydrofiner upsets the sulfur concentration in the hydrofined product can be considerably higher, e.g., as high as 50 ppm, or greater.
In reforming, sulfur compounds, even at a 1-2 parts per million weight range contribute to loss of catalyst activity and C.sub.5.sup.+ liquid yield, particularly with the new sulfur-sensitive polymetallic catalysts used by refiners in recent years. Since the late sixties, in particular, polymetallic metal catalysts have been employed to provide, at reforming conditions, improved catalyst activity, selectivity and stability. Thus, additional metallic components have been added to the more conventional platinum catalysts as promotors to further improve, particularly, the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, tin, and the like. In the use of these catalysts it has become essential to reduce the feed sulfur to only a few parts per million by weight, wppm. For example, in the use of platinum-rhenium catalysts it is generally necessary to reduce the sulfur concentration of the feed well below about 2 wppm, and preferably below about 0.1 wppm, to avoid excessive loss of catalyst activity and C.sub.5.sup.+ liquid yield. By removing virtually the last traces of sulfur from the naphtha feed, catalyst activity and C.sub.5.sup.+ liquid yield of high octane gasoline can be significantly increased.
The sulfur-containing feed, prior to reforming, is hydrofined over a Group VI-B or Group VIII catalyst, e.g., a Co/Mo catalyst, and a major amount of the sulfur is removed. Residual sulfur is then generally removed from the naphtha feeds by passage through a "sulfur trap," guard chamber, or reactor which contains a fixed bed of catalyst, or adsorbent through which the feed is passed to remove residual amounts of sulfur. Within the sulfur trap, e.g., residual sulfur is removed from the naphtha feeds by adsorption over copper chromite, nickel, cobalt, molybdenum, and the like. These and other metals have been found useful per se, or have been supported on high surface area refractory inorganic oxide materials such as alumina, silica, silica/alumina, clays, kieselguhr, and the like. Massive nickel catalysts, or catalysts containing from about 10 percent to about 70 percent nickel, alone or in admixture with other metal components, supported on an inorganic oxide base, notably alumina, have been found particularly effective in removing sulfur from naphtha feeds, notably naphtha feeds containing from about 1 to about 50 ppm sulfur, or higher.
Albeit it is known to remove sulfur from sulfur-containing hydrofined feeds by contacting, or flowing such feeds in liquid phase through a sulfur trap containing a catalyst composite constituted of nickel supported on alumina at elevated temperatures, there nonetheless remains a need for further improving the sulfur capacity and removal rate of such catalyst composites.